A. Anti-idiotypic antibodies have been used to immunize mice against infection with African trypanosomes. Administration of anti-idiotypic antibodies prior to infection appear to prime for the expression of idiotype bearing, parasite specific clones. This is the first demonstration that anti-idiotypic antibodies can substitute for antigen in the induction of anti-microbial immunity. B. The role of antibody in the susceptibility of mice to Leishmania infection was studied in Mu-suppressed B cell depleted BALB/c mice. Unexpectedly, these mice displayed dramatically enhanced resistance to infection with L. tropical and L. mexicana, suggesting that the high antibody titers which accompany chronic leishmaniasis may play a regulatory role. c. The factors controlling macrophage mediated killing of different Leishmanias were studied. Strains of Leishmania from patients with chronic muco-cutaneous leishmanasis were relatively resistant to killing when compared with healing strains. In addition, temperature significantly influenced macrophage mediated killing and may be responsible for the chronicity of certain forms of cutaneous leishmaniasis. To further study these phenomena, the parameters of lymphokine production by lymphocytes from Leishmania killing has been developed.